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Let There Be Light: Stained Glass and the Late Medieval Climate Transition Chris Simmons Supervised by Dr. Lawrence Mysak Centre for Climate and Global Change Research (C2GCR) Report No. 2007-1 McGill University, July 2007 Contents 1 Introduction 3 2 A Brief Summary of the Climate Transition of the Middle Ages 7 3 General Characteristics and Trends for Medieval Stained Glass 17 3.1 Glass Production and Colouring . 17 3.2 The Evolution of Glass Colouring: Silver Stained and Flashed Glass . 19 3.3 The Role of Enamels and their Influence on Corrosive Weathering . 21 3.4 Lead Cames and Ironwork . 25 4 French Medieval Stained Glass: A Bold Evolution from Early to Late Gothic 26 4.1 Abbé Suger’s Revolution at St. Denis: The New Style and the Transformation of Glass . 29 4.2 The Early to Mid Thirteenth Century and the Evolution of the Full Colour Tradition . 33 4.3 Early to Mid Thirteenth Century Grisailles: Providing a Lighter Interior in High Gothic France . 42 4.4 The Colour Saturation between 1240 and 1260: Ste. Chapelle, St-Germain-des-Pres, Le Mans, and Tours . 48 4.5 The Grisaille Revolution: The Development of the Band Window and the Expansion of Grisailles . 51 4.6 The Fourteenth Century through the Sixteenth Century: The Continuation of Translucency in France . 59 1 5 English Stained Glass: the Early Importance of Translucency in a Cloudier Climate 61 5.1 Canterbury, York, and the Full Colour Tradition in England . 65 5.2 The First Half of the Thirteenth Century: The Predominance of Grisailles in England . 68 5.3 Later Trends in English Glass . 72 6 German Stained Glass: A Colourful Transition to the Little Ice Age 74 7 Mediterranean Stained Glass 85 8 Criticisms and Conclusions 90 Glossary of Fenestration Terms 96 Guide to Architectural Periods and Terms 99 Acknowledgments 100 References 101 2 1 Introduction Stained glass has always played an essential role in the framework and aesthetics of Gothic architecture, from the very debut of the early style at the Basilique St. Denis to its Neo- Gothic revival in the Nineteenth century. In fact, it is difficult to conceptualize a Gothic structure without some form of ornamental glass, be it simple white grisailles with intricate leadwork or full colour windows. This choice in glass, however, not only provides an important artistic element to the building but also directly influences the amount of indoor lighting and thus affects how the of the interior structure is perceived by an observer. Additionally, in an age (such as the Medieval period) without substantial artificial lighting, besides that provided by certain lamps, candles, torches, lanterns (all often costly to maintain over long periods of time), and open doors, the amount of interior lighting available (for both functional and aesthetic purposes) is largely determined by the light that can be transmitted through windows (often referred to as daylighting). In turn, the amount of daylight that enters a building depends largely on whether the regional climate is predominately sunny or cloudy, with different regimes producing different interior lighting conditions inside Medieval churches and perhaps affecting the type of glazing that is locally preferred. This study concerns the possibility that the evolution of stained glass design in different regions during the course of the Medieval period might be related to climate and climate change. In general, Romanesque cathedrals, due to their possession of the bulky walls needed in order to construct strong, tall vaults, were often unable to support extensive window space. Under the sunnier conditions that would have prevailed during the Medieval Warm Period (MWP), however, limited window size would not have necessarily been a problem. The invention of the Gothic style toward the end of the MWP, with its external vaulting, allowed for the dramatic 3 expansion of window size over the wall space. At the same time, however, most Gothic churches were decorated with low transmissivity stained glass with deep, rich, relatively dark colours, suggesting a continued dependence on sunlight for interior lighting (thus not at once revolutionizing the interior lighting conditions from that of the Romanesque period in the manner that Wachs (1964) claims). However, as the Medieval Warm Period came to a quick close and the Little Ice Age (LIA) began over the two successive centuries, windows became more symmetric on the north and south aisles and size and acreage in cathedrals continued to increase (a response to diffuse lighting conditions, according to Wachs (1964)), while at the same time stained glass became lighter coloured and more translucent, suggesting that indirect, diffuse lighting associated with cloudier conditions became the primary source of interior illumination. The eventual elimination of age-old traditional architectural features by the Late Middle Ages, including blind triforia and tribune galleries, and their replacement with windows also suggests this demand for more light (Janson 316). Despite this trend in Northern Cathedrals, southern churches adopted the new Gothic style as well but often maintained much smaller window sizes than those of the north (indeed, the bright sunshine so prevalent in the Mediterranean climate regime, according to Wachs, Street, and Sherrill, would not be able to support such expansive windows), and when stained glass was used in the south (usually over windows no larger than Early Gothic sizes), it often possessed lower transmissivities similar to Early Gothic stained glass in the north (Assisi, Siena Duomo, Florence Duomo, Santa Maria Novella). In general, it has already been noted by Moore (3) that “generally speaking, in climates where daylight is plentiful and predictably bright, architects have responded by decreasing opening sizes or using a diffusing medium in openings, (grills, and translucent or tinted 4 glazing),” and the smaller windows with often heavy tracery (especially in rose windows) so prevalent in the Mediterranean seem to conform to this idea. A connection between regional averages in stained glass transmissivity and generalized climate regimes has also already been proposed by previous authors in the field of stained glass, with Charles Sherrill, an early Twentieth century stained glass historian and author of a famous series of guide books on stained glass, being the staunchest advocator of climate as the most important factor behind stained glass design. In addition, Wachs (1964) has also argued that the expansion of window sizes during the Gothic era in northern European churches acted as a climatic adaptation to generally diffuse lighting conditions (associated with extended periods of cloud cover) inherently present over Western Europe, made possible finally with the invention of Gothic. Although his hypothesis regarding larger windows associated with diffuse lighting can be generally confirmed by modern daylighting theory, in which modeling methods used to determine ideal natural interior lighting are separated into cloudy climates and sunny climates (Robbins 30-35), and this has also been discussed by Noal (123). However, Wachs failed to take into account the very low transmissivity of early Gothic windows, which did not increase interior lighting by the drastic amounts implied, and both authors did not consider the possibility that the climate regime had not remained constant during the period that they were considering. However, the climate relationship is still worth exploring, especially concerning the possibility that the choice of predominately coloured or white windows was partially influenced by changes that occurred in the climate during the High and Late Middle Ages. In order to understand this hypothesis linking window size and transmissivity to climate change, however, it is necessary to have some knowledge of the translucency of coloured and clear glass. Fortunately, some particularly revealing preliminary quantitative work has already 5 been done regarding general window brightness associated with different eras of stained glass. When testing a panel from the Trinity Chapel in Canterbury (Early Gothic, late Twelfth and early Thirteenth century stained glass), Robert Sowers demonstrated using an illuminance meter that the amount of light passing through a piece of stained glass is largely determined by its colour (Sowers Blues 220). For example, he discovered that red glass was the most opaque, producing a brightness of only one-half footcandle. Blue was similarly dark, with a brightness of one footcandle; green, yellow, and pink followed blue, and white was the brightest glass at five foot candles (220). This corresponds well with what humans generally perceive as bright versus dark (or rich) colours, and the observed data follows the scale that many stained glass historians refer to when judging glass as particularly bright or dark. In addition, Sowers also took similar measurements in All Saints in York (provided with a largely Fifteenth century glazing program), in which he discovered a lighter ruby at 10 footcandles, a dark blue of 13 foot candles, a light blue of 50 footcandles, and an unpainted white with 200 footcandles, thus demonstrating that the brightness of the glass tended to change with time as well as location (220). One advantage to Sowers’s study is his close proximity to the glass when making these measurements (thus demonstrating the amount of light received in one direction). However, Sowers’s research is severely limiting because it only provides data for a couple of glass

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